fix json-compilation-database pch file path bug

[BillBai]

Found a bug in json_compilation_database.rb about pch file.

When building a target A, which depends on target B. And the Target B has a prefix header and sets the precompile prefix header to YES. Target A also has its own prefix header, but sets the precompile prefix header to NO.

First xcodebuild will build target B, sets the @pch_path in JSONCompilationDatabase via format_process_pch_command, and everything is fine.

And then it begins to build target A, since the target A sets the precompile prefix header to NO, the format_process_pch_command will not be called, so the @pch_path remains the same as target B. That's the problem! the target A's pch path will be replaced by Target B's.

My solution is simply reset the @pch_path before every target in format_build_target.

[supermarin]

Thanks for the contribution @BillBai ! Mind adding a test (spec/ and/or feature/) so we ensure this doesn't break again?